关于数据库dmv的问题

SELECT TOP 5 total_worker_time/execution_count AS [Avg CPU Time],
    SUBSTRING(st.text, (qs.statement_start_offset/2)+1,
        ((CASE qs.statement_end_offset
          WHEN -1 THEN DATALENGTH(st.text)
         ELSE qs.statement_end_offset
         END - qs.statement_start_offset)/2) + 1) AS statement_text
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
ORDER BY total_worker_time/execution_count DESC;
选择占用cpu最多的前5位查询
中substring中是什么意思，有些理解不了
谢谢